1. Field of the Invention
The present invention relates generally to the removal of sulfur oxides and other contaminants contained in a flue gas by spray drying or dry scrubbing. More particularly, it relates to an apparatus and method to advance the existing dry scrubbing technology by increasing both sulfur oxide and other contaminant removal efficiencies and reagent utilization to approach those achievable by wet scrubbing, i.e., by reducing spray down temperatures to at/or near saturation while still producing a dry product.
2. Description of the Related Art
The reduction of sulfur oxides and other contaminants contained in a flue gas formed during the combustion of coal/sulfur-bearing fossil fuels and waste materials is of the utmost concern to industry. These fuels are burned by electric power generating plants, waste recycling plants, and other industrial processes. There have been many attempts to comply with federal and state air pollution requirements which have included the following methods.
One attempt has been to locate and utilize fossil fuels lower in sulfur content and/or other contaminants. The major disadvantage with this approach is the increased fuel and freight cost due to supply and demand and/or proximity to the end user.
Another attempt is the reduction of the sulfur content and other contaminants in the fuel prior to combustion by way of mechanical and/or chemical processes. The major disadvantage with this approach is the cost effectiveness of the mechanical and/or chemical processing necessary to meet the required reduced levels of sulfur and/or other contaminants.
Another approach has been the injection of dry pulverized alkali directly into the hot combustion gases for removal of sulfur oxides and other contaminants by way of chemical adsorption or absorption followed by oxidation. The major disadvantages with this approach is the low to moderate removal efficiencies, poor reagent utilization, increased particulate loadings in the flue gas which necessitate the requirement for further flue gas conditioning (i.e., humidification or sulphur trioxide injection) when the injection process is conducted upstream of a collection device such as an electrostatic precipitator (ESP).
Still another approach has been spray drying chemical absorption processes, i.e., dry scrubbing, wherein an aqueous alkaline solution or slurry is atomized via a mechanical, dual fluid, or rotary type atomizer and sprayed into a hot flue gas stream to remove sulfur oxides and other contaminants. The major disadvantage with this approach is the limitation on spray down temperature, i.e., the approach to the flue gas saturation temperature, due to the economics associated with the materials of construction and operation of downstream equipment which in turn directly limits removal efficiencies and reagent utilization.
Yet another approach has been wet chemical absorption processes, i.e., wet scrubbing, wherein the hot flue gas is typically washed with an aqueous alkaline solution or slurry to remove the sulfur oxides and other contaminants. The major disadvantages with this approach are the loss of liquid both to the atmosphere due to the saturation of flue gas and mist carryover and the sludge produced in the process. Additionally, the economics are not favorable considering the materials of construction which include the absorber module and all related equipment downstream such as primary/secondary de-watering and waste water treatment sub-systems.
U.S. Pat. No. 4,623,523 to Abrams et al discloses a confined zone dispersion technique using a boundary slip stream layer of untreated flue gas. However, this patent fails to teach the injection and control of a treated flue gas or heated air as a boundary layer with measures to actively control, modify and establish the boundary layer.
Thus, there is a need for a high efficiency advanced dry scrubber (HEADS) which advances the existing dry scrubbing technology so that the pollutant removal efficiencies are increased and reagent utilization optimized by reducing spray down temperatures to at/or near saturation similar to wet scrubbing yet producing a dry product.